1. Field of the Invention
This invention relates to a method of directly producing Ni-Ti intermetallic compounds from a lamination of Ni foils and Ti foils by using reactive diffusion.
2. Related Art Statement
Since Ni-Ti series alloys exhibit various performances in accordance with their chemical compositions, they are advanced to be put into practical use as a material for wide applications.
Heretofore, sheet or wire of Ni-Ti series alloy has been produced through steps of melting.fwdarw.hot rolling.fwdarw.cold rolling.fwdarw.intermediate annealing.fwdarw.cold rolling.fwdarw. . . . final product likewise usual metallic materials.
However, the production by the above method is very difficult owing to the inherent properties of the Ni-Ti series alloy as mentioned below. For this end, a powder sintering method has been developed instead of the above method. According to the later method, Ni powder and Ti powder are mixed at a ratio corresponding to the final composition ratio and then the mixed powder is shaped into an objective final form or a form similar thereto by a shaping technique such as HIP, CIP, cold powder rolling or the like, which is subjected to a high temperature sintering to obtain a single phase Ni-Ti series alloy through reactive diffusion of Ni and Ti. In this method, the yields at the composition adjustment and the intermediate step are considerably improved as compared with the aforementioned method of from melting to cold working.
The above two methods are a general method used for the production of Ni-Ti series alloy at the present. As another method solving the problems of the above methods, for example, Japanese Patent laid open No. 59-116340 proposes a method wherein Ni and Ti are closely adhered to each other through a film forming method such as pressing, plating, vapor deposition or the like and heated and then reaction-diffused to obtain Ni-Ti phase.
In Japanese Patent laid open No. 62-120467, there has been proposed an improvement of the method described in the aforementioned Japanese Patent laid open No. 59-116340 on the wires of Ni-Ti series alloy, wherein plural composite wires each obtained by covering a surface of Ti core with Ni are bundled together, subjected to working for reducing the size and further to diffusion treatment to produce Ni-Ti phase. This method is sufficiently practical as a method of producing the wire. Furthermore, it is possible to produce a strip of about few mm to few cm by drawing the resulting wire.
As mentioned above, there are proposed various production methods on Ni-Ti series alloys, which have many problems to be solved as mentioned below. A main cause is a point that the atomic ratio of Ni to Ti in the Ni-Ti series alloy exhibiting useful properties is restricted to about 1:1 and the cold workability is very poor as compared with the usual metallic materials.
For example, in the production steps consisting of melting.fwdarw.hot rolling.fwdarw.cold rolling.fwdarw.intermediate annealing.fwdarw.cold rolling . . . final product generally used for obtaining plate-like Ni-Ti series alloy, the combination of cold rolling and intermediate annealing steps should be repeated to a considerable extent for working to a given thickness. Such a repetition of continuous working--softening annealing causes occurrence of edge tear in the rolling, decrease of yield due to oxidation and pickling loss or the like in the annealing pickling, degradation of properties due to the oxidation in the annealing and the like, so that the productivity of Ni-Ti is poor and the cost is too high. Particularly, the production of sheet product through cold working is industrially impossible on a composition of Ni-Ti series alloy containing not less than 50 at % of Ni required for development of super elasticity at low temperature. Since such Ni-Ti alloys are difficult in the working, the wires being relatively easy in the working have mainly been produced in great amount, and the production quantity of sheet product is very small.
As a large factor obstructing the productivity to raise the cost, there is mentioned the difficulty of smelting into adequate composition. For example, in the shape-storing material, it is most important to control the actuating temperature to a given value, but in case of Ni-Ti alloy, the actuating temperature is varied to 10.degree. C. even by the change of Ni concentration of 0.1%. Therefore, the accurate composition adjustment is necessary, but since Ti has very high activity at high temperature and is lost through oxidation loss, reaction with mold or the like in the melt casting, it is very difficult to adjust the composition to the given value. As a result, special equipment is required for the melting, which obstructs a feature that the alloys having a constant quality are cheaply produced with a good yield.
As a method of avoiding various problems through the above smelting and cold working, powder sintering method has been developed. In this method, however, Ti powder which is hardly produced and is expensive should be used, so that the product cost is too high. Therefore, the powder sintering method provides a somewhat advantage when being applied to the production of parts having a complicated shape or various kinds of parts in a small quantity, but is not suitable for the production of products such as plate and strip which should be stably and cheaply be supplied in a certain large amount. In addition, since the surface of powder used as a starting material for a powder sintered body is oxidized to a certain extent, a significant amount of oxide remains in the inside of the final product, which has a problem in the quality of the product.
Furthermore, Japanese Patent laid open No. 59-116340 proposes a method wherein plate and strip are cheaply produced as compared with the powder sintering method by using the same reactive diffusion principle as in the powder sintering method. In this case, when this method is applied to the actual production of plate and strip, if it is intended to obtain Ni-Ti alloy plate of single phase having a thickness of about 0.1 mm, it is required to conduct a diffusion heat treatment for a long time such as several hundred hours. In case that the thickness of each of Ni and Ti layers is thick, defects such as voids or the like are frequently generated in the inside of the plate during the diffusion heat treatment to injury the soundness of the structure, so that the thickness produceable as a practical material by this method is only about several ten micronmeters at most. Thereofre, the latter method can not be said to be practical as an industrial production method.
Moreover, Japanese Patent laid open No. 64-31938 discloses a method considered as an extension of the above production method. According to this method, the material is not particularly limited to Ni-Ti, but plural layers of foil-like metallic material are laminated and then subjected to a heat treatment to conduct diffusion.
In this method, however, the reactive diffusion is solid-phase diffusion, so that in case of the reactive diffusion between Ni and Ti, particularly solid-phase reactive diffusion of flatly laminated Ni-Ti plate as compared with the case of using powder as a starting material, there are caused peculiar problems as mentioned below, and consequently it is difficult to obtain members having a practically usable quality and also the treating time becomes long. Moreover, these problems have been confirmed from the experimental results of the inventors.
(1) A first point is a time required for the reactive diffusion. Considering the same weight, the interface area proceeding the diffusion (specific surface area: mm.sup.2 /g) is less as compared with the powder, so that the long time is required for the progress of the diffusion. PA1 (2) A second point lies in that voids are frequently generated by Kirkendall effect as a phenomenon inherent to the interactive diffusion because the absolute number of atoms passing per unit interface area is increased by the same cause as mentioned above. Particularly, in case of interactive diffusion between Ni and Ti, the diffusion rate of Ni atom in Ti is larger by 1000 times or more than the diffusion rate of Ti atom in Ni, so that Ni atom largely tends to reduce in the vicinity of the interface, and consequently the occurrence of Kirkendall voids becomes conspicuous. The occurrence of voids not only injures the structure but also obstructs the subsequent reactive diffusion at the interface to interfer the homogenization of the composition, so that it is required to reduce the occurrence of the voids as far as possible. Moreover, the occurrence of voids is closely related to the heat treating temperature of reactive diffusion. In case of Ni-Ti, the occurrence of voids can be controlled to a certain extent at a relatively low temperature of about 700.degree. C., but the diffusion rate becomes later and the homogenization of the composition takes a long time and it is unpractical. On the other hand, the heat treatment is carried out near to an upper limit temperature for solid reaction of about 900.degree. C. in order to shorten the reaction time, but in this case a large amount of voids is generated. PA1 (3) A third point is a phenomenon resulting from a difference in interactive diffusion rate between Ni and Ti, in which the increase and decrease of volume is caused with the advance of diffusion between Ni and Ti layers and hence stress is produced at the interface to cause the mechanical peeling phenomenon. When this point is further explained in detial, the volume naturally tends to relatively reduce in the Ni layer preferentially discharging atom, and in this case such a tendency of volume reduction is macroscopically developed as a decrease of layer thickness in the thickness direction. On the other hand, in the Ti layer absorbing Ni atom, the layer thickness macroscopically increases and also the layer expands in the plane direction. Therefore, the shearing force acts in the plane direction in the vicinity of the interface between Ni and Ti layers, and consequently the mechanical peeling is caused at the interface. PA1 (a) subjecting the laminate to a rolling for thickness adjustment in which the laminate is rolled at a draft of not less than 30% to join laminated foils to each other and then rolled to render a thickness of each foil into 3-20 .mu.m; PA1 (b) subjecting the rolled laminate to a preliminary sol id phase diffusion heat treatment at a temperature of 650.degree.-780.degree. C. for 1-10 hours; PA1 (c) subjecting to a solid phase diffusion heat treatment at a temperature of 781.degree.-955.degree. C. for 1-10 hours; PA1 (d) subjecting to a first liquid phase diffusion heat treatment at a temperature of 956.degree.-1110.degree. C. for 10 minutes to 1 hour to render only a portion having a Ti content of more than 50 at %; PA1 (e) subjecting to a second liquid phase diffusion heat treatment at a temperature of 1111.degree.-1240.degree. C. for 1 to 10 minutes to render only a portion having a Ni content of more than 50 at %; and PA1 (f) finally subjecting to a soaking heat treatment at 1000.degree.-1100.degree. C. for 1 to 10 hours.
From the above reasons, it is said that the production method described in Japanese Patent laid open No. 64-31938 is not practically applicable in industrial scale.
Although the production method described in Japanese Patent laid open No. 62-120467 can be said to be sufficiently applicable to the industrial production of wire and strip, the size of the resulting article is naturally critical because the size of original wire and strip is restricted, so that this method articles having various sizes such as thicker and wider products can not be produced.